While it is known that acid-modified or anhydride-modified polymers are useful as adhesives for bonding to both metal and polyolefins, there is a need to have adhesives with not only high-bond strength to both metal and polyolefins, but also to activate the bonding at relatively lower temperatures. Conventional and currently commercially available modified polyolefins sacrifice either bonding strength to the metal or to the polyolefin, or do not activate at low temperatures when bonding to metals or polyolefins. Furthermore, adhesive compositions that purport to activate at low temperatures are often very soft and tacky materials such that the handling of these compositions as free films in thermal laminating processes is problematic and may require very costly release films to prevent blocking. Also, most current adhesives tend to significantly lose their bond strength over time after they are put into service. This phenomenon is referred to as “age-down” in the industry.
There are number of patents in the art that disclose adhesive compositions but these prior art patents are not satisfactory in every aspect mentioned in this specification. Examples of these prior art patents include U.S. Pat. Nos. 5,516,583; 4,861,676; 4,861,677; 4,552,819; and 5,965,255.
One example of the prior art is U.S. Pat. No. 5,225,482 issued Jul. 6, 1993 to Nakagawa et al, which discloses an adhesive composition comprising an ethylene-vinyl acetate copolymer, a styrene polymer resin, a graft-modified polyethylene, a polystyrene elastomer and an ethylene-α-olefin copolymer. There is no disclosure of the use of high impact polystyrene as the styrene resin in combination with an olefin/non-olefin copolymer.
It has been found that the formulations for the adhesive compositions of the present invention solve many, or all, of these problems. The adhesive compositions of the present invention bond to both metal and polyolefins, activate at relatively lower temperatures and are easily handled as a free or coextruded film not requiring interleafing or a release paper. In addition, it has been found that these adhesive compositions fail 100% cohesively during the peel test. Cohesive failure is a desirable attribute in that it can be an indicator of high bond strength, so much so that the bond strength is greater than the cohesive strength of the adhesive. Cohesive failure also provides a convenient visual test to assure that multi-layer constructions are properly bonded when other testing methods are not readily available. Finally there appears to be a strong correlation between the cohesive failure mode of the adhesive and the retention of bond strength after the product is put into service. Accordingly, the present invention provides adhesive compositions that possess excellent bonding properties to metallic substrates and to a number of polymeric materials resulting in an adhesive layer with high peel strength. The present invention also allows for a relatively lower activation temperature to be used in the manufacture of building panels.
The adhesive compositions of the present invention may be supplied as an adhesive resin in pellet form or as an adhesive film.